As is well known, a spark plug generates spark discharge for ignition at a discharge gap between a center electrode and a ground electrode. The shapes of the center electrode and the ground electrode have been adaptively changed in various ways in accordance with the intended use and required properties of the spark plug. In particular, there has been known a spark plug in which a plurality of ground electrodes are provided so as to realize improvement of fouling resistance and ignition performance, lowering of a voltage required for discharge (required voltage), etc. For example, see Japanese Patent Application Laid-Open (kokai) No. S60-081784 (“Patent Document 1”); Japanese Patent Application Laid-Open (kokai) No. H05-326107 (“Patent Document 2”); Japanese Patent Application Laid-Open (kokai) No. H08-315955 (“Patent Document 3”); Japanese Patent Application Laid-Open (kokai) No. 2001-237045 (“Patent Document 4”); Japanese Patent Application Laid-Open (kokai) No. 2005-183189 (“Patent Document 5”); and Japanese Patent Application Laid-Open (kokai) No. 2008-171646 (“Patent Document 6”).
A spark plug having a plurality of ground electrodes has a problem in that if the shape and positions of the ground electrodes are improper, spark is deflected by a flow of gas around the discharge gap, and so-called multiple discharge occurs, or generation of multiple discharge cannot be restrained. If multiple discharge occurs, consumption of the electrodes is accelerated, whereby the service life of the spark plug becomes shorter.
An object of the present invention is to provide a technique for reducing the occurrence of multiple discharge in a spark plug.